Many products, such as beer and other malt beverage products, dairy products and real juices, must be packaged in such a way that oxygen cannot migrate into the package before the package is opened to permit consumption of its contents; otherwise, over the normal shelf life of the filled package oxygen will degrade the flavor of its contents. Heretofore, such products, when packaged in glass containers, or, more recently in plastic bottles, have been capped with a closure, such as an aluminum roll-on closure or a molded plastic closure, that is lined with an internal liner that functions both as a sealing liner and, to a lesser extent, an oxygen-barrier liner. Commonly-assigned U.S. Pat. No. 4,721,221 (Barriac), the disclosure of which is incorporated by reference herein, discloses a molded plastic closure with a molded plastic closure with a sealing liner, this reference teaching a top seal liner for pressurized beverage products. In either case, the liner must sealingly engage the rim of the associated container, either on its top or both on its top and side, to property seal the filled and capped container.
In recent years, there has been a concerted effort to eliminate the need for inserting a sealing liner in a molded plastic closure to eliminate the expense relating thereto. To that end, self-sealing molded plastic closures have been developed, and U.S. Pat. No. 5,638,972 (Druitt) and U.S. Pat. No. 5,836,464 (Druitt) the disclosure of each of which is also incorporated by reference herein, teach unlined, molded plastic closures of a general type that has proven to be quite successful in the packaging of carbonated soft drink products, which, though somewhat less sensitive to the migration of CO2 out of the product, are not particularly sensitive to the migration of oxygen into the packaged product. However, such closures, as heretofore used in the packaging of carbonated soft drinks, are not sufficiently oxygen-impermeable to permit their use in the packaging of beer and other malt beverage products, and other oxygen-sensitive products, when such products must undergo a normal shelf life between packaging and opening for consumption.